Re: [PATCH v3 0/8] make slab shrink lockless

[Qi Zheng]

On 2023/3/1 02:40, Michal Hocko wrote:
> On Mon 27-02-23 17:08:30, Mike Rapoport wrote:
> [...]
> > The results you present do show improvement in IPC for an artificial test
> > script. But more interesting would be to see how a real world workloads
> > benefit from your changes.
>
> It's been quite some time ago (2018ish) when we have seen bug report
> where mount got stalled when racing with memory reclaim. This was
> nasty because the said mount was a part of login chain and users simply
> had to wait for a long time to get loged in in that particular
> deployment.
>
> The mount was blocked on a shrinker registration and the reclaim was
> stalled in a slab shrinker IIRC. I do not remember all the details but
> the underlying problem was that a shrinker callback took a long time
> because there were too many objects to scan or it had to sync with other
> fs operation. I believe we ended up using Minchan's break out from slab
> shrinking if the shrinker semaphore was contended and that helped to
> some degree but there were still some corner cases where a single slab
> shrinker could take a noticeable amount of time.
>
> In general using a "big" lock like shrinker_rwsem from the reclaim and
> potentially block many unrelated subsystems that just want to register
> or unregister shrinkers is a potential source of hard to predict
> problems. So this is a very welcome change.

Totally agree. :)

Thanks,
Qi